This invention generally relates to optical modulation. More particularly, it relates to a device for high data rate modulation of an optical signal. Even more particularly, the invention relates to a device that provides a high amplitude and high data rate of magneto-optical modulation.
Because of the high data rates available, optical fiber is preferred for high-speed transmission of data, audio, and video. Binary optical signals consist of low and high intensity signals traveling through the fiber. The limiting factor has been the speed at which light can be electrically switched or modulated to provide change from high intensity signal to low intensity signal and back to high intensity signal. This conversion from electrical to optical is slower than the capability of the fiber. While the optical fiber can accommodate much higher data rates, commercial techniques for creating high-speed modulation are presently achieving approximately 40 billion bits per second, or 40 GHz.
One method provides a digital optical signal directly from a light source. In this method signal is directly modulated by turning on and turning off power to a laser source of light, but it is difficult to make these transitions quickly without introducing non-linear effects which degrade the signal. These effects include changes in index of refraction of material in the laser cavity which effectively changes the optical path length of the cavity during the pulse, leading to an effect called chirp, and provides greater dispersion of the signal as it travels down optical fiber.
Alternatively, a continuous wave light source can be externally modulated to create a desired digital optical signal. One method is electroabsorption modulation. Continuous wave light is directed through a semiconductor. When current flows in the semiconductor, enough electrons are moved from valence to conduction band to provide a population inversion. Light traveling through the semiconductor with the population inversion is amplified by stimulated emission. On the other hand, when no electric current flows, electrons move back to the valence band. Now the light is absorbed, so the light intensity is diminished as it travels through the semiconductor. The substantial difference in light intensity when current is flowing and when current is not flowing provides the on- and off-signals. However, this scheme is limited by the time for generation and relaxation of excited states in the semiconductor.
A third method, a Mach-Zehnder modulator, provides another external modulation technique in which a light beam traveling in a waveguide is split into two paths and then recombined into a single path where the two beams interfere. A material is provided along one path that has an index sensitive to applied voltage. The change in phase introduced by the changing voltage applied to the material provides for constructive or destructive interference where the signals recombine. Currently, however, 10-15V is needed to provide the phase shift, and a problem has been to make high frequency signals at a high voltage to drive the phase modulator.
An alternative approach to increase the amount of data that can be transmitted through an optical fiber is Dense Wave Division Multiplexing (DWDM), in which many individual signals, each with a slightly different wavelength, are transmitted through a single optical fiber at one time. Each of the dozens of signals in the fiber runs at the 40 GHz data rate, providing a substantially higher overall data rate. While DWDM increases the data rate provided by a fiber, the equipment cost for transmission capacity is higher providing additional wavelengths than is the cost by providing a faster modulation with a single wavelength. Also, errors may be introduced into the data as a result of a process known as four wave mixing, in which photons of different wavelengths in a fiber combine, so data is lost in two channels in the fiber. Two other photons are generated at different wavelengths, and these may contribute to noise and errors in other channels in the fiber. Thus, faster modulation for each wavelength is desirable.
Two additional techniques to greatly increase modulation frequency to increase the data rate for a wave in a fiber have been proposed in commonly assigned U.S. Pat. No. 5,768,002 to K. A. Puzey, and in a paper xe2x80x9cMagneto-Optical Modulator for Superconducting Digital Output Interface,xe2x80x9d by Roman Sobolewski, et al, given at the Applied Superconducting Conference held Sep. 17-22, 2000 (xe2x80x9cthe Sobolewski paperxe2x80x9d). Superconductors allow low voltage high-speed current switching.
The Puzey technique rapidly switches a superconducting film between superconducting and non-superconducting states and takes advantage of the difference in optical properties of the material in the two states. In the superconducting state more far infrared light is reflected from the material, while in the non-superconducting state, more is transmitted. Continuous far-infrared light is modulated by an electrical signal provided to such a superconducting film. After modulation of this far-infrared light, the signal is then parametrically converted to a shorter wavelength in the near-infrared range for transmission in a standard optical fiber. Well known frequency up-conversion nonlinear optics are used for the conversion.
The technique described in the Sobolewski paper stimulates magneto-optic material 10, such as europium monochalcogenides (EuS, EuTe, EuO, and EuSe) by providing magnetic field 12 from current pulse 14 in adjacent superconducting signal electrode 16 driven by a Josephson junction, as shown in FIGS. 1a, 1b. Continuous light wave 18 is coupled into magneto-optic material 10 through fiber optic input 19a and exits through fiber optic output 19a. Portion of light wave 18 traveling in magneto-optical material 10 in magnetic field 12 has its polarization rotated, a property known as the Faraday effect. An interferometer is used to provide pulses of light based on this rotation of the polarization. Because the excitation of magneto-optical materials occurs in a time measured in pico-seconds, as shown in FIG. 2a from a paper, xe2x80x9cFemtosecond Faraday rotation in spin-engineered heterostructures,xe2x80x9d by J. J. Baumberg, et al, J. Appl. Phys. 75 (10), May 15, 1994 (xe2x80x9cthe Baumberg paperxe2x80x9d), early investigators recognized that such microstriplines might provide a way to modulate signals in the THz (trillion bits per second) range, about two orders of magnitude higher than present modulation. While the Sobolewski paper discloses high speed magneto-optic modulation it is limited to small rotations (4.52xc2x0). A 4.52xc2x0 rotation limits throughput to 0.6%.
Although a number of authors have suggested advantages to modulating light based on magneto-optical materials, none suggests a scheme that provides large rotations of the polarization at a high data rate. Thus, a better system for converting an electrical signal to an optical signal is needed that provides short pulses having high amplitude, and this solution is provided by the following invention.
It is therefore an object of the present invention to provide a method of increasing power of an optical signal provided from a magneto-optical system;
It is a further object of the present invention to provide a method of increasing polarization rotation of an optical signal provided from a magneto-optical material;
It is a further object of the present invention to provide a scheme for rapidly rotating polarization of an optical signal while eliminating a slow relaxation of the polarization;
It is a further object of the present invention to provide a scheme for increasing polarization rotation and rapidly rotating polarization while eliminating a slow relaxation of the polarization;
It is a further object of the present invention to provide a scheme for rapidly modulating power of an optical signal;
It is a further object of the present invention to provide a high power optical signal generated by a magneto-optical system while delay associated with relaxation from an excited state of the magneto-optical material does not affect data rate;
It is a further object of the present invention to provide modulation at a short wavelength and to transform the modulated wave to a longer wavelength for transmission in an optical fiber.
It is a further object of the present invention to provide amplification of a modulated optical signal using an optical amplifier;
It is a further object of the present invention to provide absorption of an off-signal while allowing transmission of a portion of an on-signal to improve on-signal to off-signal ratio;
It is a feature of the present invention that multiple rotations of the plane of polarization of light are provided by stimulating the magneto-optical material multiple times;
It is a feature of the present invention that pairs of oppositely directed rotations of the plane of polarization of light are provided by stimulating the magneto-optical material with a single current pulse crossing the material twice;
It is a feature of the present invention that a superconductor is used to couple a current pulse stimulation to the magneto-optical material;
It is an advantage of the present invention that a narrow high amplitude pulse is generated; and
It is an advantage of the present invention that the optical modulating is at a much higher data rate and much higher amplitude than is otherwise achievable.
These and other objects, features, and advantages of the invention are accomplished by a method of making an optical signal comprising the step of providing a material. Incident radiation is directed at the material. The incident radiation includes a first parameter having an initial value. The incident radiation includes a first segment. A first stimulation is provided to the material to provide a first change to the first parameter in the first segment. A second stimulation is provided to the material to provide a second change to the first parameter in the first segment.
Another aspect of the invention is a method of generating an optical signal comprising the step of directing incident radiation at a material. The incident radiation comprises a first parameter having an initial value. A plurality of stimulations is provided to the material to change the value of the first parameter of radiation. A pulse of radiation is generated from the continuous radiation. The pulse of radiation comprises a second value of the first parameter. The pulse further comprises a pulse width, wherein the pulse width is shorter in time than is achievable with a single one of the plurality of stimulations or the second value is greater than can be achieved with a single one of the plurality of stimulations.
Another aspect of the invention is a device comprising a source of radiation for providing radiation having a first segment, a waveguide for modulating radiation from the source of radiation, a pulse generator, and a plurality of electrical conductors. The conductors are connected to the pulse generator with a splitter for receiving pulses in each conductor. The conductors extend across different portions of the waveguide and have delay elements that cause the pulses from the pulse generator at the waveguide to all intersect the first segment of the radiation.
Another aspect of the invention is a device for providing an optical signal, comprising a magneto-optical material. A source of incident radiation is configured to direct radiation at the material. The incident radiation includes a first parameter having an initial value. The incident radiation also includes a first segment. The device includes a first conductor for providing a first current pulse for providing a first magnetic stimulation to the material to provide a first change to the first parameter in the first segment. The device also includes a second conductor for providing a second current pulse for providing a second magnetic stimulation to the material to provide a second change to the first parameter in the first segment.
Another aspect of the invention is a method of providing a signal comprising the step of forming a light pulse comprising an on-portion and an off-portion. The off-portion has a residual magnitude. The pulse is directed through a saturable absorber to absorb residual off-signal. The pulse is amplified after the saturable absorption step.
Another aspect of the invention is a method of fabricating a device, comprising the steps of providing a substrate, forming a superconductor on the substrate, and forming a magneto-optical material on the superconductor.